1. Field of the Invention
This invention relates generally to fiber optic signal processing, and more particularly to apparatus for mixing light modes, a method for producing fiber optic mode mixers and a method for producing fiber optic mode mixing.
2. Description of the Prior Art
Transmitting light energy through a length of fiber optic tubing is analogous to transmitting energy through a waveguide. In both cases, energy reflects off the "walls" of the fiber optic tubing or the walls of the waveguide, and modes are established within the fiber optic tubing or waveguide.
FIG. 1 helps illustrate how these modes are created. A light source 10 and a light receiver 20 are coupled to end faces 30 of a length of multimodal fiber optic tube 40 having walls 50. Fiber optic tube 40 is solid, and walls 50 are the outer boundary of tube 40. A plurality of light rays 60, 60', 60", . . . 60.sup.N leave light source 10 and upon entering fiber optic tube 40 begin to reflect off walls 50. Although FIG. 1 shows only four such rays, it will be understood that an infinite number are present.
Depending upon the angle, .phi., at which light rays 60, 60', . . . 60.sup.N enter fiber optic tube 40, the radius, r, of fiber optic tube 40 and the wavelength of light source 10, certain rays will tend to reinforce one another, while other rays will tend to cancel each other. As a result, different modes are established within fiber optic tube 40 and transfers of energy from mode to mode can occur within fiber optic tube 40. The transfer of energy from mode to mode is termed modal dispersion. If radius r is made sufficiently small, only one mode can be supported and the fiber optic tube is said to be single mode. Generally radius r is sufficiently large to support many modes, and the fiber optic tube is said to be multimode. However, the energy will tend to concentrate in only a few modes, a situation exacerbated by bending fiber optic tube 40.
This energy concentration in a few modes is undesired when light source 10 supplies analog or digital information to a multimode fiber optic tube 40 because fidelity of the information is impaired. If, however, the various modes can be coupled or mixed within multimode fiber optic tube 40, a more even distribution of energy from mode to mode occurs and signal fidelity is improved. In practice, mode mixing is desirable because lengths of fiber optic tube 40 may be several Km in practice, and concentration of signal energy in only a few modes is inevitable. However if the modes comprising the light entering fiber optic tube 40 can be thoroughly mixed, energy will be evenly distributed and the undesired modal concentration effect in fiber optic tube 40 will be minimized.
The prior art recognizes that placing an optical fiber tube under deformation stress will modify various characteristics of the fiber. For instance, U.S. Pat. No. 4,389,090 issued to LeFevre discloses an apparatus and method for stressing an optical fiber to control the polarization of light passing through the fiber. A portion LeFevre's optical fiber is twisted to form at least one, generally planar, coil, thereby changing the polarization.
In the past, tubing containing lengths of fiber optic were filled with shot which tended to stress the fiber at uniform points along its length and to promote mode mixing. U.S. Pat. No. 4,178,069 issued to Kaiser, et al. discloses an optical fiber having an intentionally deformed waveguide axis to introduce mode coupling, thereby reducing modal dispersion.
However such prior art mode mixing maintains a constant radius, r, in the fiber optic tube and, as a result, produce incomplete mixing. Further, the characteristics of prior art mode mixers are not readily varied or easily reproduced.